Regulation for Water Conservancy Computation of Water Projects SL 104-95 lacks the specific methods on runoff regulation of irrigation reservoir, and the calculation method commonly used in reservoir engineering design is not reasonable to some extent. In this paper, an improved calculation method was proposed. The improved method modified the calculation method of long time series in the conventional method and applied the long series continuous calculation. It can correct the unreasonable calculation conditions in the conventional method and simplify the calculation process, and it can be applied for the irrigation reservoir with a variety of regulation performances. The improved method was verified by multiple engineering projects, and the results indicated the calculation process is reasonable and regulation results are accurate. Moreover, irrigation water and irrigation assurance increased using the improved method compare with those obtained from the conventional method,.